Chronic Total Occlusion (CTO) is a pathology where an artery (which may often be a coronary artery) is completely occluded by a build-up of plaque. It usually happens over time, and may often be treated via PI (percutaneous intervention), which may be a PCI (percutaneous coronary intervention).
During a percutaneous intervention, a guide wire may be pushed into a patient's arterial system all the way to a blockage. The blockage may comprise hard and/or soft plaque. The blockage may also be referred to as an occlusion.
Many techniques may require the guide wire to traverse the occlusion in some way. For example, the guide wire may traverse the blockage to allow the deployment of a catheter for the stenting or removal of the blockage.
FIG. 1a shows an example of an occlusion 16 in a vessel 10 having a vessel wall 12 and lumen 14. The vessel 10 is partially blocked by the occlusion 16. However, some patent lumen (unblocked lumen) is present at 18, allowing some blood to flow through the occlusion 16. Blood flow is represented by an arrow 19. The blood flow is constricted on entering the occlusion 16.
In FIG. 1b, a guide wire 20 carrying a dilation device 22 is threaded through the occlusion 16, and the dilation device 22 is used to dilate a stent 24 once the stent 24 is in position at the occlusion 16.
FIG. 1c shows the dilated stent 24 once the dilation device 22 has been removed. The stent 24 remains dilated and allows increased blood flow (represented by arrow 26) where previously the vessel was partially blocked by the occlusion 16.
Where a narrow passage through the occlusion in the artery is present (for example, as illustrated in FIG. 1), a surgeon may try to thread a small guide wire through the occlusion. The passage through the occlusion may often be small. The risk of perforating the artery or dislodging plaque and causing infarction may be considerable. In some circumstances, very little navigational information may be available.
In a case of complete occlusion, the surgeon may decide to attempt one of two techniques. In a first technique, the surgeon may push the guide wire through the blockage by navigating the wire through a soft component of the occlusion rather than hard component of the occlusion such as calcium. In a second technique, the surgeon may perform a subintimal crossing of the occlusion. In a subintimal crossing of the occlusion, the catheter creates a new channel between the intimal and medial layers of the vessel wall to bypass the plaque. The catheter should then re-enter the true lumen of the vessel. If the catheter does not re-enter the true lumen, a subintimal dissection may occur.
In some normal catheterization laboratory (cath-lab) interventions, fluoroscopy or X-ray angiography (XA) may be used to guide the progress of the guide wire. However, in the case of chronic total occlusion, visualization of an intervention may be difficult. Plaque may not be well imaged in fluoroscopy or X-ray angiography.
FIG. 2 shows an angiography image, in which vessels 30 appear near-black due to contrast enhancement of blood. The tip of a catheter is indicated by arrow 32. A guide wire 34 is shown pushing through a blockage. The material forming the blockage is not visible in X-ray angiography. Therefore, only limited navigational information may be provided by the X-ray angiography.
In some circumstances, very little information may be available regarding the composition of the material in front of the guide wire. For example, the material composition may not be apparent from an X-ray angiography image such as the image of FIG. 2.